1. Field of the Invention
The present invention relates to a novel porous article to which a sulfur containing atomic group is introduced, i.e., a sulfur containing atomic group introduced porous article, a process for introducing a sulfur containing atomic group onto outer-inner surfaces of a porous article, and a battery separator comprising the sulfur containing atomic group introduced porous article.
2. Description of the Related Art
Hitherto, a sulfonating treatment was known as one process for introducing a sulfur containing atomic group onto a surface of a porous article. The conventional sulfonating treatments include, for example, a treatment with a fuming sulfuric acid, or a treatment with a plasma produced by an atmospheric-pressure glow discharge in an atmosphere of a gaseous mixture of a treating gas and a rare gas. The sulfonation on the surface of the porous article, i.e., the introduction of sulfonic groups (SO3H) onto the surface to be treated, can impart a hydrophilic property to the porous article or enhance a hydrophilic property of the porous article. A degree of the sulfonation on the treated porous article correlates with a ratio (S/C) of the number of sulfur atoms (S) to the number of carbon atoms (C) on the treated surface (hereinafter referred to as a surface S/C ratio). A higher degree of sulfonation correlates with a higher surface S/C ratio.
The sulfonation treatment with a fuming sulfuric acid can introduce sulfonic groups at a high degree, and a treated article having a high surface S/C ratio can be obtained. Nevertheless, the sulfonated article treated in this process tends to exhibit a lower ratio (O/C) of the number of oxygen atoms (O) to the number of carbon atoms (C) on the treated surface (hereinafter referred to as a surface O/C ratio), i.e., an index to show a degree of introduction of a carboxyl group and so on. The surface O/C ratio correlates with a wettable property, or wetting property. An article with a lower surface O/C ratio has a poor wettable property. For example, when a sulfonated material obtained in this process is used as a battery separator, a further treatment (for example, an application of a surface active agent) is required to enhance the wettable property. Further, the process using a fuming sulfuric acid requires a large-scale apparatus for washing away unreacted fuming sulfuric acid, and a large-scale treatment of a waste liquor containing a concentrated sulfuric acid. There is also a problem of a deterioration in the strength of the treated article.
The treatment with a plasma produced by an atmospheric-pressure glow discharge in an atmosphere of a gaseous mixture of a treating gas and a rare gas is disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 9-85026. The publication discloses a process for treating a porous article having a three-dimensional network wherein the article is placed between a pair of electrodes which are located opposite to each other, at least one of the electrodes carrying a dielectric layer on a surface facing the other electrode; and a discharged plasma treatment is conducted under an atmospheric pressure in the presence of a gaseous mixture of a treating gas and a rare gas. The publication description also states that if a concentration of the treating gas in the gaseous mixture is beyond 10% by volume of the rare gas, i.e., a concentration of the treating gas is beyond 9.09% by volume of the gaseous mixture, it becomes difficult to generate a uniformly discharged plasma upon applying a voltage, and thus the concentration of the treating gas is preferably 0.01 to 9.09% by volume of the gaseous mixture. Further, the publication mentions that a discharged plasma treatment was actually performed under a sulfur dioxide/helium mixture containing 0.5% by volume of sulfur dioxide as a treating gas, and states that sulfur dioxide, sulfur trioxide, oxygen or nitrogen can be used as the treating gas.
As apparent from the description of the above publication, it is necessary in the process for an atmospheric-pressure glow discharge to apply a voltage in the presence of more than 90% by volume of rare gas, to generate a uniformly discharged plasma. Therefore, it is impossible to increase the concentration of the treating gas in the gaseous mixture over the limitation, and thus, a sulfonated article having a high surface S/C ratio and a high surface O/C ratio at the same time was not known.
Accordingly, the object of the present invention is to provide a novel porous article or material wherein a sulfur containing atomic group is introduced and a high surface S/C ratio and a high surface O/C ratio are exhibited at the same time.
Another object of the present invention is to provide a means whereby the sulfur containing atomic group is introduced onto the outer-inner surfaces of the porous article or material to a high degree by carrying out a discharge under an atmospheric pressure, and thus, a large-scale apparatus or a large-scale treatment of a waste liquor containing a concentrated sulfuric acid is not required.
Still another object of the present invention is to provide a battery separator having a high surface S/C ratio and a high surface O/C ratio at the same time.
Other objects and advantages will be apparent from the following description.
In accordance with the present invention, there is provided a porous article wherein a sulfur containing atomic group is introduced onto at least a part of outer-inner surfaces of the porous article, a ratio (S/C) of the number of sulfur atoms (S) to the number of carbon atoms (C) on the surface onto which the sulfur containing atomic group is introduced is 7xc3x9710xe2x88x924 or more, a ratio (O/C) of the number of oxygen atoms (O) to the number of carbon atoms (C) on the surface onto which the sulfur containing atomic group is introduced is 0.2 or more, and the ratio (S/C) and the ratio (O/C) are determined by means of an X-ray photoelectron spectrophotometer.
Further, in accordance with the present invention, there is provided a battery separator comprising a non-woven fabric wherein a sulfur containing atomic group is introduced onto at least a part of outer-inner surfaces of the non-woven fabric, a ratio (S/C) of the number of sulfur atoms (S) to the number of carbon atoms (C) on the surface onto which the sulfur containing atomic group is introduced is 7xc3x9710xe2x88x924 or more, a ratio (O/C) of the number of oxygen atoms (O) to the number of carbon atoms (C) on the surface onto which the sulfur containing atomic group is introduced is 0.2 or more, and the ratio (S/C) and the ratio (O/C) are determined by means of an X-ray photoelectron spectrophotometer.
Further, in accordance with the present invention, there is provided a process for introducing a sulfur containing atomic group onto at least a part of outer-inner surfaces of a porous article comprising steps of:
placing the porous article between a pair of electrodes which are located opposite to each other, at least one of the electrodes carrying a dielectric layer on a surface facing the other electrode; and
applying a voltage between the electrodes in the presence of 10% by volume or more of a gas of a sulfur containing compound, based on a total volume of a surface-treating gas, under an atmospheric pressure to thereby cause a discharge.
Further, in accordance with the present invention, there is provided a process for introducing a sulfur containing atomic group onto at least a part of outer-inner surfaces of a porous article, comprising steps of:
placing the porous article between a pair of electrodes which are located opposite to each other, at least one of the electrodes carrying a dielectric layer on a surface facing the other electrode; and
applying a voltage between the electrodes in the presence of a gas of a sulfur containing compound and an oxygen gas under an atmospheric pressure to thereby cause a discharge.
Further, in accordance with the present invention, there is provided a process for introducing a sulfur containing atomic group onto at least a part of outer-inner surfaces of a porous article, comprising steps of:
placing the porous article between a pair of electrodes which are located opposite to each other and carry a dielectric layer on a surface facing the other electrode, respectively, so that the porous article is brought into contact with both dielectric layers without coming into contact with the electrodes; and
applying a voltage between the electrodes in the presence of a gas of a sulfur containing compound under an atmospheric pressure to thereby induce an electric discharge in voids contained in the porous article and sandwiched between a pair of the electrodes.
The term xe2x80x9couter-inner surfacesxe2x80x9d as used herein with reference to the porous article or material collectively indicates (i) an outer surface of the porous article or material to be treated, and (ii) an inner surface of the porous article or material to be treated. The term xe2x80x9couter surfacexe2x80x9d means a surface which is in contact with a hypothetical or virtual smooth surface of a hypothetical or virtual geometrical solid hypothetically circumscribing the porous article or material with the hypothetical smooth surface. The term xe2x80x9cinner surfacexe2x80x9d means a surface of a void contained in the hypothetical or virtual geometrical solid hypothetically circumscribing the porous article or material with the hypothetical smooth surface. Namely, the inner surface of a foamed porous article or material denotes the surfaces of all cells, the inner surface of a porous film denotes the surfaces of concave portions, such as depressions or grooves, or through-holes, and the inner surface of a fabric porous article or material denotes the surfaces of the inner spaces formed by constitutional fibers, that is, the surfaces of the constitutional fibers.
The term xe2x80x9csulfur containing atomic groupxe2x80x9d as used herein means an atomic group which contains one or more sulfur atoms and newly introduced onto a porous article or material to be treated, by treating the porous article or material in accordance with a process of the present invention. The conditions of the sulfur containing atomic group existing in or binding to the porous article or material are not limited. The sulfur containing atomic group includes, for example, an atom, a functional group, a crosslinked group, a molecule, or the like which physically and/or chemically bonds to the porous article or material. The sulfur containing atomic group introduced onto the porous article or material treated, i.e., the sulfur containing atomic group on the sulfur containing atomic group introduced porous article or material, can be determined by means of an X-ray photoelectron spectrophotometry.